PhotoVoltaic (PV) solar panels are an important source of power for the electrical grid. Large, megawatt PV installations with PV panels numbering in the tens of thousands are increasingly common. PV panel arrays are typically organized into panel “strings” with each string consisting of from 10 to 20 PV panels connected in series. In a Direct Current (DC) PV panel system, the output of a PV panel string could connect to a central inverter which converts the DC power of the PV panels into AC power suitable for the electrical grid. Typical PV panel string voltages at the inverter input may range from 500 V to 1000V.
A typical PV panel is organized as a series connection of individual PV cells. A common configuration is 72 PV cells per panel. A typical PV cell operating voltage under illumination is about 0.7 V. An illuminated 72 PV cell panel will therefore have an output voltage of about 50 volts DC.
A known problem of connecting PV panels and PV cells in series is uneven photocurrents produced by individual PV cells. Uneven photocurrents may be caused by one or more of: shading of a particular PV cell or group of PV cells in a PV panel, shading of an entire PV panel in a multi-panel system, soiling of a portion of a PV panel or an entire PV panel in a multi-panel system, differences in PV panel orientation relative to the illumination source, and PV cell/panel manufacturing variations. Since all PV cells in a PV panel are series connected, the current output of the PV panel is limited by the PV cell with the lowest photocurrent. In the case of a heavily shaded PV cell, a large reverse bias voltage may be generated across it by the remaining unshaded PV cells, forcing current through the shaded PV cell. This reverse bias voltage may become large enough to cause a catastrophic breakdown of the PV cell and create permanent damage.
A common solution to this problem is to add anti-parallel bypass diodes in parallel with groups of PV cells. A common arrangement for a 72 PV cell panel is to have three “sub-strings” of 24 PV cells each, with one bypass diode per sub-string.
Bypass diodes prevent formation of high reverse bias voltages. The bypass diode is normally reverse biased. However, when a portion of the PV cells in a sub-string become sufficiently shaded, for example, the bypass diode will become forward biased and conduct the photocurrent of the remaining unshaded sub-strings, preventing damage to the PV cells in the shaded or partially shaded sub-string.
Bypass diodes have a number of disadvantages. For instance, they are prone to failure. If the failure is an open circuit failure, then the bypass diode's PV cell sub-string is no longer protected and the PV panel may suffer catastrophic failure. If the bypass diode failure is a short circuit failure, then the PV cell sub-string to which the failed diode is attached will not produce useful power. Another disadvantage of bypass diodes is their power dissipation. Bypass diodes decrease PV panel efficiency by consuming power whenever they become forward biased. A bypass diode has a forward bias voltage of about 0.7 V. With a typical PV panel current of 8 A, each forward biased bypass diode dissipates 5.6 W. If a PV panel that includes three PV cell sub-strings and three bypass diodes were heavily shaded and its three bypass diodes were to become forward biased, then the fully bypassed PV panel would actually represent a load, dissipating 16.8 W. With one and two PV cell sub-strings bypassed, the forward biased bypass diode(s) would reduce the power output of this example PV panel by 5.6 W and 11.2 W respectively.